* Because you have one and want to play your games on it.
* Because porting from Android is easy (and for commercial, you don't need as many additional sales to cover the cost of porting).
* Because even if sales aren't as good as the other consoles, you might benefit from less competition, or getting in there early (and if it is popular - which is very possible given the low price - all the better, and for some it's worth taking the risk now).
* Because you want to write for consoles, but don't want to deal with the barriers usually associated with console development.
* Because you want to support "open" platforms.
Personally I like PC/laptop gaming and wonder why people bother with consoles, but that's an argument against any console, and evidently many people like consoles. I think one benefit seems to be better controllers that are available and supported as standard.
@Hodgman: Presumably you have to worry about using a controller rather than touchscreen, so although porting may be easy from a code point of view, it's not quite yet another Android phone/tablet.
@MrJoshL: I generally agree with you, but some nitpicks:
"(similar to the early days of smartphones with iOS leading the pack"
Incidentally, for sales IOS never led the smartphone pack (Symbian number one until 2011, Android since then). I wouldn't call 2008 early days at all (that was more like 2000 - by 2005, even bog standard mainstream cheap feature phones did Internet, games and apps), but several other smartphones outsold IOS too (BlackBerry, even Windows Mobile) ( http://en.wikipedia.org/wiki/Smartphone#Historical_sales_figures
). Although yes, people were happy to develop for IOS despite sales, either because of perceived future growth, or other reasons - so all the more reason that I don't see the reason to criticise people developing for any other new platform (especially one that's really just a slight variant of an already massively popular OS).
"Android is messy."
It's less friendly and with less options to develop for than say Windows desktop, but how does it compare to other consoles like PS3? Also, these issues haven't stopped people developing games for phones and tablets in general. I think using Android is better than trying to create new APIs, as even though they might be easier, it would be extra effort for those who already know or develop for Android. I guess the obvious alternative choice would have been GNU/Linux, but I can see them being tempted by the large pool of Android developers.
I agree about low storage - in general, I'd argue that this is the biggest thing holding back phone/tablet games. They're more than capable of reasonable 3D games, but such high end games typically take ~1GB of storage, so people don't have room to install many, and most games end up with simpler graphics. When competing with consoles, this difference may be more unacceptable to users.
It supports USB though, I wonder if this would support storing applications on an external disk?
If you understand why people may buy it, surely that answers why people may develop for it? Same argument as for any other platforms really: